This phase I randomized, double-blind, multicenter comparative clinical trial will compare the effectiveness of 5% and 10% nonoxynol-9 in a 2 gram long-acting, sustained release suppository system, administered intravaginally at 5-15 min and 6-8 hours before coitus, in preventing penetration of spermatozoa into mid-cycle cervical mucous. Signs and symptoms of irritation will also be compared between the two products. PSA analysis will also be performed to obtain information about the use of this marker as an indication for intercourse. Healthy sterilized women ages 18-45 with regular menstrual cycles will be recruited. At the screening visit, consent will be obtained, a medical history, breast and pelvic exams will be performed. The subject will be taught to use an ovulation predictor kit and instructed to call on the first day of her period. Subjects will be scheduled at midcycle for a cervical mucous check and if this shows evidence of ovulation, will return the next day for post-coital testing within three hours of intercourse. Each subject will have a baseline cycle and two test cycles (one for each time interval). Colposcopy will be performed during the baseline cycle exams.